How do you scroll the amount of loopcuts/bevel segments in edit mode without having to change the middle button to scroll?
After pressing Ctrl+R, you can use PgUp to increase and PgDn to reduce the number of loop cuts. Using this you can adjust the amount to fit your needs dynamically, while entering a number can be faster if the amount of cuts needed is known.
Updated:
The Num + and Num - as @moonboots mentioned and the above mentioned PgUp and PgDn work for me in Blender version 2.83.3, which should be the version* used if you do not work with the experimental ones.

*version used currently as it's the stable and also LTS version at the time of this post.